1. Field the Invention
This invention is directed to a detector associated with a paper feeding mechanism, in general, and to a detector which signals that a paper feeding tray or cassette is empty, in particular.
2. Prior Art
With the advent of the computer age and, more importantly, the personal computer explosion, more and more uses are being found for printing devices. The devices are used as an adjunct to the computing system to produce the hardcopy output.
As computers achieve higher operating speeds, the printing devices operate faster, as well. In many instances, the printing device is a laser printer which is capable of printing a large number of sheets in a very short period of time. For example, it is not at all unusual to find laser printers which can print 8 to 16 pages per minute. This equates to a page every four to eight seconds. Obviously, higher speed capablities are permissible and expected.
In conjunction with many such printers, the paper to be printed upon is provided in trays or cassettes. The trays or cassettes permit the printer to be used with different types of paper, different sizes of paper and so forth. That is, one cassette may be used for 81/2.times.11 paper, another cassette may be used for 81/2.times.14 paper and so forth. The use of the cassettes permits quick and easy interchange and replacement of paper in the printer.
Of course, the trays or cassettes tend to have a relatively limited amount of paper therein. Consequently, with the high-speed printing capabilities of the printing apparatus, the cassette must be monitored and the supply of paper replenished. In the event that the paper is not replenished in a timely fashion, a great deal of inefficient operation of the computer can be encountered because of having to retrace the steps of the machine to determine when printing terminated (because of lack of paper) and to reinitiate the cycle. Because of the high speeds noted above, it is sometimes difficult to monitor the paper supply in the cassette. In addition, because of the high speed of operation, it is difficult to keep the paper supply replenished while conducting other activities with regard to the computing system. Consequently, it is highly desirable to have a means for detecting when the paper in the tray or cassette has run out. There are many techniques known for other types of printing devices, such as printing presses or the like. These devices usually indicate a break in the paper which is mechanically detected. These types of detectors usually operate to shut down the system automatically. These systems are usually operative in terms of a continuous web of paper being passed through the system wherein a tension detection device is utilized.
However, such systems are usually quite expensive and highly interruptive in the printing process because of shutting down the entire system. Moreover, the cost involved in such a device is usually prohibitive relative to the costs of the computer and printer systems used in most personal computer applications.
It is, therefore, highly desirable to have some alarm system which indicates the status of the paper supply in the paper tray or cassette. With respect to smaller-size printers, there are specific detector units or devices. For example, some printers include visual indicators which light or flash when the paper supply cassette is empty. Unfortunately, this indicator is frequently overlooked by the printer operator.
Some very sophisticated devices include a software package which keeps track of the paper supply and warns the operator when the supply is low or exhausted.